Electrical resonators are used in many electronic applications. For example, acoustic resonators are an enabling technology for radio frequency (RF) mobile communications. In many wireless communications devices, RF as well as microwave frequency resonators are used as filters to improve reception and transmission of signals. Acoustic resonators act as low loss and high fidelity filters for the many frequency bands upon which modern mobile communications are based. Filters typically include inductors and capacitors, and more recently resonators. As applied in RF filtering for mobile communications, acoustic resonators typically require stable and accurate frequency tuning, high Q values, and low nonlinearities.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) resonator. The BAW resonator has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The BAW includes an acoustic stack. The acoustic stack includes, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
Film bulk acoustic resonators (FBAR) filters are a form of BAW filters. FBAR filters are state of the art for acoustic resonators. FBAR technology is characterized by superior performance in terms of Q over frequency, effective coupling coefficient kt2, and precise frequency control. These FBAR performance characteristics translate to superior product performance in terms of (low) insertion loss, (satisfactory) roll off characteristics at filter edge, (optimum) isolation, and (highest) nonlinearity performance.
FBARs include a piezoelectric layer sandwiched between two metal electrodes, i.e., a top metal electrode and a bottom metal electrode. FBARs are placed above an air cavity, and rely on air cavity packaging technology to achieve required performance characteristics. As a result, air cavities below FBARs must necessarily be robust and must not interfere with the resonator frequency centering, Q values, or nonlinearities.
Many BAW resonators, including FBARs, include a substrate made of a semiconductor material, such as silicon. Electromagnetic fields generated in the acoustic stack, for example, can induce currents in the semiconductor substrates. These induced currents themselves generate electromagnetic radiation that can interfere with the desired electrical signals of the resonator and can degrade the performance of devices (e.g., filters) that incorporate the resonator. For example, these induced currents can contribute to intermodulation distortion (IMD), which is a measure of linearity for a wide range of radio frequency (RF) and microwave components. Fundamentally, IMD describes the ratio (in dB) between the power of fundamental tones and third-order distortion products, and it is desirable to reduce IMD products, which adversely impact desired linear behavior of devices (e.g., electrical filters) that incorporate BAW resonators.
In an effort to reduce currents induced by electromagnetic fields in the vicinity of the semiconductor substrates of BAW resonators, and thereby reduce IMD and other sources of distortion, semiconductor substrates used in BAW resonators are often made from undoped, highly resistive material. While some improvements are realized using undoped, highly resistive materials for the substrate, third order IMD products (IMD3) remain problematic especially as higher power transmission and reception, and reduced channel spacing, drives the communications industry.
Packaging for FBAR(s) may include a semiconductor microcap lid placed over the FBAR(s) and the above-noted air cavities formed below the FBAR(s). The microcap lid may be held above the FBAR(s) by posts that are formed from the same material as the microcap lid and that are integral with the microcap lid wafer. The microcap lids are a wafer-level silicon cap (microcap) micromachined from a high resistivity wafer.
Parasitic contributions from the microcap lid wafer/microcap lid degrade linear performance characteristics of the packaged FBAR product. The parasitic contributions arise from the bulk conductivity of the silicon material, as well as surface capacitances and inversions, plus the charging and discharging of semiconductor trap states. Any solution to the parasitic contributions that provides high performance at a lower cost should not interfere with the air cavities below the FBAR, nor adversely affect the frequency centering, Q values, or nonlinearities.
Additionally, gold thermocompression bonds may be used to hold the posts to the substrate that includes the air cavity below the FBAR. The posts are aligned with the gold thermocompression bonds to affix the microcap lid wafer over the FBAR device wafer. The gold thermocompression bond is expensive. At least in part to due to this expense, the size of the area in which the posts are attached to the substrate is limited, and the gold thermocompression bond is used only around a perimeter of the posts. Additionally, the gold thermocompression bond is used at high-pressure/high-temperature which poses a potential obstacle to scaling FBARs to wafers larger than, for example, 200 millimeters. Moreover, the gold thermocompression bonds are IR-based rather than lithography based.
Finally, singulation scribes (air gaps) are present in the silicon microcap in the region between the FBAR base wafer and the silicon microcap, in the scribe regions. In the silicon microcap device, these air gaps require difficult singulation technologies that may often result in perimeter chip-out which must be screened by testing in the final product.